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EQUAD NEWS WINTER 2007
Engineering Security: Securing the Internet Using physics to secure future wireless networks Rapid advances in wireless technology are quickly moving us toward a pervasively connected world in which a vast array of wireless devices, from iPhones to biosensors, seamlessly communicate with one another. A National Research Council report of several years ago, titled “Embedded, Everywhere,” concluded that so-called ubiquitous networking “could well dwarf previous milestones in the information revolution.” This revolution is now becoming a reality—in November Reuters reported that roughly half the world’s population owns a cell phone—yet the unique security challenges that these new pervasive communication networks pose have yet to be satisfactorily resolved. Because it may be impractical to secure these networks through conventional means of encryption alone, H. Vincent Poor and his team of researchers at Princeton are investigating ways to ensure security by exploiting the physics of the wireless medium in which these networks operate. “Encryption works well today in existing networks—however, new types of network architectures may make conventional encryption techniques very difficult to implement,” said Poor, the Michael Henry Strater University Professor of Electrical Engineering and dean of the School of Engineering and Applied Science. “While new ideas for using encryption in such networks are also being studied, we are looking at the possibility of using properties of the physical wireless medium—the so-called ‘physical layer’—itself to help overcome these limitations.” Poor is a leading figure in the field of wireless communications, and his group at Princeton has produced a considerable amount of scholarly output applying the principles of information theory to understand the potential of the wireless physical layer for providing security in wireless networks. This year alone, Poor and his collaborators—including, in addition to his own research group at Princeton, colleagues at Bell Labs and at several other universities in the United States and abroad—have presented more than a dozen papers addressing various aspects of this approach at leading international conferences. While cryptography is a very old field—Caesar himself used encryption to communicate military secrets—information theory has been around for little more than a half century. Claude Shannon, an electrical engineer and mathematician, established the field when he set forth the elements of digital communications in his landmark 1948 paper, “A Mathematical Theory of Communication.” At the time, Shannon himself did not think the field held much promise for security, but researchers have subsequently proved otherwise. One of Shannon’s most influential ideas is that communication channels have a fundamental “capacity” that characterizes the rate at which data can be reliably transmitted through them. Poor’s work is based on the notion of “secrecy capacity,” which similarly characterizes the rate at which data can be transmitted both reliably and securely. Other physics-based approaches to communications security have been used successfully and commercially in other contexts, notably in communication over optical fibers. How can physics aid in securing a wireless network? One way is to exploit a phenomenon called “fading.” Wireless signals moving from transmitter to receiver typically do not travel by a single direct path. Instead, a signal originating from the cellular base station in Cleveland Tower on the Princeton campus, for example, might ricochet off the Woodrow Wilson School’s Robertson Hall and then bounce off a bus on Olden Street before rebounding to an intended recipient sitting in the EQuad Café. The signal that eventually reaches the receiver will be a composite of signals that arrived over multiple such paths. This signal scattering causes fluctuations in the received signal, or fading—an essential property of wireless networks that distinguishes them from communication networks operating over fiber optic cables, copper wires or other physical media. Fading has traditionally been considered to be an obstacle to providing clear wireless communication. But over the past decade, it has been demonstrated that fading can actually aid in providing additional capacity and reliability in wireless networks. Some of Poor’s research in this area is directed at seeking ways in which fading can also help provide security. “Fading offers new opportunities for communicating securely in an environment in which an intended receiver and a potential eavesdropper experience independent fading fluctuations,” Poor said. “The transmitter can exploit those variations in the channels to communicate securely with the intended receiver.” Although much of Poor’s research tends to be close to practice (in the past year he has received three U.S. patents, all of which have been assigned to industry), at this point his information theoretic security work is still, well, theoretical. However, the insights gained from theoretical contributions have often shaped future technologies, and information theory has provided a particularly rich conceptual basis for wireless technologies. “We’re not going to patent this today and build it tomorrow,” said Poor. “But many wireless technologies have sprung from fundamental research like this. We are now beginning to shift the focus of this research to more practical issues, and our hope is that this effort will soon yield practicable techniques that can be applied to emerging wireless networks.”
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